This project deals with a) the structural requirements for thymic independence in the induction of an antibody response, and b) the role of T lymphocyte function in the switch from IgM to IgG antibody production: a) This problem is being approached by assessing the thymic dependence of fixed, multimeric antigens which are normally thymic dependent in the unfixed state. Results so far indicate that 1) mice will respond to their own haptenated red cells at a level equivalent to their response to haptenated heterologous red cells, and 2) that fixed, haptenated red cells are unsuitable for these studies. Research is currently being carried out with other multimeric systems. b) We are currently assessing the effect of graft vs. host reactions on the class of antibody produced in response to a thymus independent antigen. Results so far indicate that the presence of an ongoing GVH reaction elicits a significant switch from a response favoring IgM production to one favoring IgG production. Work for the coming year will deal mainly with a) the thymic dependence of fixed, haptenated keyhole limpet hemocyanin and glutamic dehydrogenase vis a' vis the thymic dependence of their unfixed forms, and b) a further assessment of the effect of GVH reactions on the class of antibody produced, wherein several parameters will be investigated.